The invention relates to a modulator comprising a first differential amplifier having a first input for receiving an input signal and a second input for receiving a correction signal for adjusting a quiescent current on a first and a second output of the first differential amplifier; a second differential amplifier and a third differential amplifier, whose control inputs are each coupled to one of the two outputs of the first differential amplifier, having interconnected difference inputs for receiving a carrier signal and having interconnected difference outputs for supplying a modulated signal, and a control amplifier having an input coupled to means for at least temporarily applying to this input a difference signal which is proportional to the difference between the current on the first and the current on the second output of the first differential amplifier and having an output coupled to the second input of the first differential amplifier via a storage device.
Such a circuit arrangement is known from European Patent Specification No. 0,040,274 and may be employed in, for example, a video recorder in order to convert a chrominance signal from a higher to a lower frequency band during recording or from to a lower to a higher frequency band during reproduction. The chrominance signal comprises two signal components, the colour signal and the burst signal, which recur periodically and between which a signal gap occurs.
The first differential amplifier comprises two transistors whose emitters are coupled to a current source. The input signal is applied to the base of the first transistor and a correction signal is applied to the base of the second transistor. The collector of the first transistor is connected to the emitter of a third and the emitter of a fourth transistor, constituting the control input of the second differential amplifier, and the collector of the second transistor is connected to the emitter of a fifth and the emitter of a sixth transistor, constituting the control input of the third differential amplifier. The carrier signal is applied between the base of the third and the base of the fourth transistor. The base of the third and that of the fifth transistor and the base of the fourth and that of the sixth transistor are interconnected. The modulated signal is taken from the coupled difference outputs of the second and the third differential amplifier, i.e. the collector of the third and that of the fifth transistor and the collector of the fourth and that of the sixth transistor are interconnected.
For the conversion of the chrominance signal from a lower to a higher frequency band it is necessary that the quiescent currents of the collector of the first transistor and the collector of the second transistor be substantially equal in order to prevent the signal from being influenced by the subcarrier. The collector quiescent current of the first or the second transistor is the current which flows in the absence of an input signal.
In the known circuit arrangement the quiescent currents are corrected as follows. The control input of the second differential amplifier is connected to a first switching transistor and the control input of the third differential amplifier is connected to a second switching transistor. The two switching transistors are also connected to a control amplifier which is coupled to the base of the second transistor via an earthed capacitor. The differential amplifier supplies a correction signal for adjusting the quiescent currents. The switching transistors are cut off during the occurrence of the horizontal pulses, because there is a signal gap in the chrominance signal during this time. In order to guarantee that the switching transistors conduct only during the occurrence of the horizontal pulses a control circuit is necessary for controlling the switching transistors.
The control amplifier only generates a correction signal during the time in which the horizontal pulses appear, which correction signal is stored in the capacitor. Therefore, the capacitor is generally so large that it cannot be integrated when the modulator is constructed as an integrated circuit.